Truck bombs killed at least 27 people and wounded nearly 450 in attacks on the British consulate and a London-based bank today - days after a bloody terrorist strike on Istanbul synagogues.

The worst terror bombings in Turkey's history coincided with US President George W. Bush's trip to Britain and were blamed on al-Qaeda.

Security forces were on highest alert after the blasts at the high-rise headquarters of the HSBC bank and the British consulate, which occurred with 10 minutes of each other at about 11am local time. Turkey said suicide bombers carried out the attacks.

Witnesses said one pickup truck exploded just outside the HSBC building, while another truck crashed into the gate of the British consulate destroying annexes to the main consular building.

"The man came, rammed into the gates," Interior Minister Abdulkadir Aksu told the Anatolia news agency, describing the consulate bombing. The attacker then "set it off, blowing himself up".

The vehicle looked like a catering truck, with explosives placed in food cauldrons, Anatolia quoted police as saying.

"Cars were flying, there was blood everywhere, I could see fruit and vegetables flying in the air from stalls," said Mehmet Dag, 26, who works for a textile company as a delivery man. He was about 100 metres away from the consulate building.

The blasts followed a pair of synagogue bombings on Saturday that killed 23 people, plus the two bombers. Turkish authorities blamed the attacks on the same groups.

Turkish Premier Recep Tayyip Erdogan vowed that Turkey will defeat terrorists and he deplored the timing of the attacks, during the Islamic month of Ramadan.

"Those who bloodied this holy day and massacred innocent people will account for it in both worlds," he said. "They will be damned until eternity."

"The terrorists hope to intimidate, they hope to demoralise. They are not going to succeed," Bush said at a news conference with Blair.

British Foreign Secretary Jack Straw was scheduled to arrive in Istanbul this evening. Earlier, he suggested a link to al-Qaeda.

"I'm afraid it has all the hallmarks of international terrorism practiced by al-Qaeda," he said in London.

An unidentified caller to the semiofficial Anatolia news agency said al-Qaeda and the militant Islamic Great Eastern Raiders' Front, or IBDA-C, jointly claimed responsibility for attacks.

A schoolgirl cries in front of the British consulate.Photo: AFP

The Turkish central bank said it was taking steps to prevent financial fallout from the attacks, and would intervene to stop fluctuations in the Turkish currency. Turkey's benchmark index dropped about 7 per cent until the stock exchange was closed.

In London, the Foreign Office said it would send a rapid deployment team of consular staff and other officials to Istanbul. The BBC reported 16 officers from the British police's anti-terrorist squad were heading to Turkey to help the investigation.

"The reaction for the moment is sadness and grief but carrying on with what we're here for," Peter Westmacott, British ambassador to Turkey told Cable News Network. He said Short, 58, "will be terribly missed".

Local staff of the British consulate were also among the victims, including a cook and security guard, Anatolia said. Rescue officials said their was little hope of finding anybody alive in the rubble.

The double attack was Turkey's worst terrorist bombing and its highest single-day death toll from terrorism since 1977, when gunmen opened fire on leftists celebrating May Day, killing 37 people.

The first Istanbul blast was at the Turkish headquarters of HSBC, the world's second-largest bank, shearing off the facade of the 18-storey building and shattering the windows of nearby skyscrapers in the affluent district of Levent.

Body parts, the charred shells of cars and broken glass were scattered around a three-metre-deep crater in the street outside the bank. Water gushed out of the top floors of the building.

Bystanders, bloodied and covered in dust, looked dazed as they walked past lines of ambulances. Several people helped carry the limp bodies of victims. Road signs laid on the ground, twisted next to fallen tree trunks.

The second bomb ripped off the wall surrounding the garden of the British consulate in the downtown Beyoglu district, a popular tourist destination with shops, bars, movie theatres and restaurants. The US consulate was moved months ago to a more secure location in another district.

Television reports initially spoke of up to five blasts, but Turkish authorities later confirmed only two.

"Once again we are reminded of the evil these terrorists pose to people everywhere and to our way of life," Blair said. "Once again we must affirm that in the face of this terrorism there must be no holding back, no compromise, no hesitation in confronting this menace."

An American witness was walking near the British consulate when the attack happened.

"I heard a series of explosions. I thought it was echoing. You could see smoke and smell sulphur in the air," Tim Little, a 44-year-old American tourist from Grand Lake, Colorado.

One witness was travelling on a bus near HSBC when the explosion occurred.

"I thought somebody hit our bus from the back, then I saw black smoke rising. Cars were damaged all around us. I saw the charred body of a driver at the wheel," said a sobbing Mehmet Altan.

Witness Hakan Kozan, 29, who was close to the British consulate said the blast came from a white pickup truck. "I heard a slam on the brakes and 10 seconds later the explosion came," Kozan said.

Mehmet Celik, who was slightly injured, said a light brown pickup truck "exploded in front of the HSBC headquarters".

In a sign of international concern over safety in Turkey, Europe's soccer authorities postponed two international soccer matches scheduled to be played in the country next week, including a game between Maccabi Haifa of Israel and Spain's Valencia which had already been switched from Israel because of security fears.

The Istanbul State Security Court imposed restrictions on news coverage of attacks, but Turkish TV stations continued their broadcasts from the scenes.

Turkish army troops made a brief appearance on the streets in Istanbul, deploying on a major highway and standing guard beside police in Istanbul.

The deployment was a significant step, since the military is a powerful force that leads the secular establishment in this predominantly Muslim country. The military took power here three times between 1960-80, and in 1997 it forced a religious-oriented government out of power without staging a coup.

In the synagogue bombings, authorities arrested six people yesterday. A Turkish court charged five with "attempting to overthrow the constitutional structure", which carries a sentence of life imprisonment. The sixth was charged with "helping illegal organisations", punishable by five years in prison, Anatolia said. No trial date was set.

The two suicide bombers who attacked the synagogues in pickup trucks were identified as Turks. Foreign Minister Abdullah Gul said the two had visited Afghanistan in the past and that investigators were pursuing al-Qaeda links.

On Sunday, Osama bin Laden's terror network claimed responsibility for the bombings in messages to two Arabic-language newspapers, but it was not possible to authenticate those claims. The outlawed Turkish radical group IBDA-C also claimed responsibility, but Turkish authorities said the attack was too sophisticated to be carried out by that group.

One of the synagogues, Neve Shalom, had been attacked twice in the past. In 1986, gunmen killed 22 worshippers in an attack blamed on radical Palestinian militant Abu Nidal. The Turkish Islamic militant group Hizbollah - not linked to the Lebanon-based group - bombed the synagogue in 1992, but no-one was injured.